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How To Correctly Size A UPS Battery

UPS batteries are sized in ampere-hours (Ah), referring to the amps they can supply per hour. The battery within a UPS must be sized to provide a specified runtime (also known as battery autonomy), in minutes or hours, at a specific load.

Load-specific factors are incorporated into a general battery sizing algorithm that will also take into account a number of other issues relating to ambient temperature and the inverter and system efficiencies of the UPS being considered.

A battery set may comprise of a single battery string or multiple strings connected in parallel. A battery string consists of one or more battery blocks connected in series, with the ampere-hours (Ah) rating of the blocks used to define the total Ah of the battery set.

Whilst battery manufacturers provide discharge curves and tables to assist selection, UPS manufacturers generally have bespoke battery sizing programs designed around their own UPS characteristics.

Their battery packs also tend to be somewhat uniform, based around battery ranges and their source of battery cabinets or stands.

For any given load, the runtime period must be sufficient to keep the connected loads running, allow for the start-up of standby generators (both automatic and manual if necessary) and provide sufficient time for the completion of automatic load shedding or shutdown routines.


Most Important Factors To Consider When Sizing UPS Batteries:

  • Site power history, and in particular, the duration of previous mains power supply failures
  • Critical load sizes
  • Whether or not there is a maintained standby generator on site
  • The length of time required for load shedding and/or to carry out an orderly shutdown of equipment
  • The installation environment, for example, available floor space, ambient temperature and humidity
  • Cost versus space requirements for longer runtime systems.


Comparing battery autonomy between different UPS manufacturers is often tricky. The most effective method is to ensure all runtimes are quoted using the same load Watt ratings and load power factors.

As an example, for a 10kVA UPS supporting an 8kW load at 0.8pF, the runtime from a 12Ah internal battery set would be approximately 14 minutes. However, at 75% load (6kW) and 0.6pF, the same UPS would provide autonomy of almost 20 minutes. 

For more information, or support sizing your UPS battery system, contact our sales team.

Alternatively, we've developed an online UPS Configurator to help you find the right UPS for your load.


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Lifespan of UPS batteries

Batteries are essential for UPS correct functioning. In this whitepaper we explain the main important factor that affect their duration and we provide some recommendations for optimizing the UPS + batteries system, in order to maximize its lifespan and general availability.